X(3), X(4), X(1316), X(2698), X(2782)
four foci of the MacBeath inconic : X(3), X(4) and two imaginary
This isogonal focal is the locus of foci of inscribed conics centered on the line X(5)X(39). See also K164.
K166 = nK(X6, X3569, X3).
The singular focus F is X(2698).